This invention relates to computed tomography (CT) systems and specifically to CT systems in which projections of the imaged object are taken both along rays within the gantry plane and rays crossing the gantry plane.
In a typical computed tomography system, an x-ray source, mounted to a rotating gantry, is collimated to form a fan beam with a defined fan beam angle. The fan beam is typically oriented to lie within the "gantry plane", a plane normal to the axis of rotation of the gantry, and is transmitted through an imaged object to an x-ray detector array also oriented within the gantry plane. The axis of rotation of the gantry is also referred to as the z-axis.
The detector array is comprised of a line of detector elements, each of which measures the intensity of transmitted radiation along a ray projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The x-ray source and detector array may be rotated on the gantry within the gantry plane and around a center of rotation so that the "gantry angle" at which the fan beam axis intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the collected intensity signals from each detector element. The gantry is then rotated to a new angle and the process is repeated to collect projections data along a number of gantry angles to form a tomographic projection set.
Often, 2.pi. radians or 360.degree. of gantry rotation will be used to collect the projection set; however, for fan beam CT systems, it has been determined that a mathematically complete projection set may be obtained with as little as .pi. radians, plus the angle of the fan beam of gantry rotation. The use of less than 2.pi. radians of gantry rotation to collect a projection set will be referred to generally as "half scan".
The acquired tomographic projection sets are typically stored in numerical form for later computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art. A projection set of fan beam projections may be reconstructed directly into an image by means of fan beam reconstruction techniques, or the intensity data of the projections may be sorted into parallel beams and reconstructed according to parallel beam reconstruction techniques. The reconstructed tomographic images may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
A typical computed tomographic study involves the acquisition of a series of "slices" of an imaged object, each slice parallel to the gantry plane and having a slice thickness dictated by the size of the focal spot, the width of the detector array, the collimation, and the geometry of the system. Each successive slice is displaced incrementally along a z-axis, perpendicular to the x and y axes, so as to provide a third spatial dimension of information. A radiologist may visualize this third dimension by viewing the slice images in order of position along the z-axis, or the numerical data comprising the set of reconstructed slices may be compiled by computer programs to produce shaded, perspective representations of the imaged object in three dimensions.
As the resolving power of computed tomography methods increases, a growing number of slices are required in the z-dimension. The time and expense of a tomographic study increases with the number of sequential slices required. Also, the longer scan times necessary to acquire more slices increases the discomfort to the patient who must remain nearly motionless to preserve the fidelity of the tomographic reconstructions. Accordingly, there is considerable interest in reducing the time required to obtain a slice series.
One method of decreasing the scanning time needed to collect multiple slices of data is to acquire projection data for more than one slice during a given gantry rotation. This may be done by using a two-dimensional detector array extending along the z-axis to obtain projection data on either side of the gantry plane, and by changing the collimation of the x-rays from that of a fan beam to, for example, a cone beam having rays diverging from a focal spot not only within the gantry plane but to either side of the gantry plane as well. It will be recognized that such a cone beam generally need not be a true cone but may also include, for example, pyramidal dispersions of x-rays in three dimensions. The collection of radiation from more than a single plane during one projection will be referred to generally as three dimensional scanning.
Despite the potential advantages of three-dimensional scanning, the images are frequently degraded by artifacts which obscure structures within the imaged object.